Self-diffusion and viscosity of low molecular weight polystyrene over a wide temperature range

The self-diffusion coefficient D and the viscosity eta of low molecular weight polystyrene (1.9 X 10(3) g/mol) have been measured from T-g + 3 degreesC to T-g + 160 degreesC. Forward recoil spectrometry (FReS) and pulsed-gradient spin-echo NMR methods were combined to determine D values covering more than 10 orders of magnitude. Over this range, D has the same temperature dependence as T/eta, to within 1 decade. The small difference can be explained by the different temperature dependences of segmental and terminal relaxation near T-g. The product of D and the terminal relaxation time is independent of temperature over the entire temperature range to an excellent approximation. In contrast, the analogous quantity for low molecular weight glass-forming liquids can increase by several orders of magnitude as T-g is approached from above. This difference is interpreted in terms of the length scale of the spatially heterogeneous dynamics which develop near T-g.